The present invention relates to a fluidized bed boiler and a method of operating same, and more particularly to such a boiler and method in which an acceptor is introduced into the fluidized bed for capturing the sulfur generated during the combustion process.
Fluidized bed reactors or boilers have long been recognized as an attractive and effective means of generating heat when used as a gasifier, combustor, or the like. In these arrangements air is passed through a bed of particulate material which normally consists of a mixture of inert material, a particulate fossil fuel, such as bituminous coal, and an acceptor, such as limestone, used for the capture of sulfur generated during the gasification or combustion of the fossil fuel. The air fluidizes the bed and promotes the combustion of the fuel resulting in a combination of high heat release, improved heat transfer to surfaces within the bed and compact reactor or combustor size.
In these types of arrangements, it is highly advantageous to use a calcined limestone, normally referred to as "lime", since, if calcined, the lime is 30% to 50% more effective in capturing the sulfur from the combusted fossil fuel when compared to raw limestone that has not been calcined.
Although it is possible to calcine the limestone directly within the fluidized bed, the reaction is usually completed less efficiently due to the temperatures and conditions that must be maintained within the bed which results in reduced reactivity for most limestone acceptors. In addition, breaking up of the limestone particles into very fine particles occurs on shock heating, with these fine particles being carried away from the bed with the mixture of air and gaseous products of combustion. These effects, of course, also reduce the effectiveness of the acceptors.
According to some prior art techniques, the raw limestone can be calcined externally of the fluidized bed, or purchased in a calcined form, before it is introduced into the bed. However, since calcined limestone costs approximately eight to ten times more than raw uncalcined limestone, it can be appreciated that this can considerably add to the cost of the process.